A thruster as here understood is such a propulsion device of a marine vessel that is formed of at least a propeller unit situated, at least in its operating position, beneath the hull of the marine vessel. The thruster may be a steerable one, a retractable one or a stationary one. The drive of the propeller may be arranged mechanically, hydraulically or electrically. Though the present invention covers all three drive options, the following exemplary description of the thruster concentrates on the structures required by the mechanical drive as it requires the most complicated mechanical arrangements. The electric and hydraulic drives have been only briefly discussed.
The exemplary thruster, when viewed from the standpoint of the lubrication has three main parts, i.e. the upper gearbox, the vertical shaft, and the lower gearbox. The upper gearbox includes the upper gear transmission that is formed of an angle gear transmitting power from a substantially horizontal drive shaft to the substantially vertical shaft. The upper gearbox forms normally an independently lubricated unit. The vertical shaft is normally formed of three parts, i.e. of a shaft extending downwardly from the upper gearbox, of a floating intermediate shaft, and of a pinion wheel shaft extending into the lower gearbox. The intermediate shaft may be coupled to the upper gearbox shaft and to the pinion wheel shaft with flexible or floating shaft couplings or the intermediate shaft may be replaced with a flexible or floating shaft coupling. The lower end of the vertical shaft, i.e. the pinion wheel shaft transmits the power to a substantially horizontal propeller drive shaft located within the lower gearbox.
If the thruster has an electric or hydraulic drive, the upper gearbox of the mechanical drive may be replaced with the electric or hydraulic drive. The shaft of the electric or hydraulic drive motor is vertical and connected, preferably by means of a flexible or floating coupling, to the intermediate shaft or directly to the pinion wheel shaft. The electric or hydraulic drive motor may sometimes be provided with a shaft extending down to the pinion wheel to form its shaft, too. A yet further option is a so-called e-pod, where the electric drive motor is coupled directly to the propeller drive shaft.
Since the thruster discussed in this specification may, optionally, be a steerable one, such a thruster has to be made rotatable round the vertical axis. This means that the upper gearbox has to be kept stationary, while the rest of the thruster components are steered, i.e. rotated round their substantially vertical axis. To fulfil this requirement the upper gearbox is fastened by means of an annular cover plate to the hull structure of the marine vessel. The cover plate is provided with means for supporting the rotating part of the thruster in vertical direction, i.e. a set of bearings, and means for rotating the thruster, i.e. a set of gearwheels and at least one motor for rotating the gearwheel. The hull structure below the cover plate is provided with a downwardly tapering shell that, by means of the bearings arranged at its lower end supports the thruster in horizontal direction. Both the upper and lower bearings as well as the gearwheel require lubrication, whereby the tapering shell forms a closed cavity called stembox. As the gearwheels and the upper bearings are located just below the cover plate, their proper and reliable lubrication is a challenging task unless it has been arranged by means of full-bath lubrication. Thus, the stembox is also provided with sealing at its lower end. The stembox is needed only when it is a question of a rotary, i.e. steerable, thruster. In non-rotary thrusters the stembox is replaced with a thuster frame.
Below the stembox there is another cavity, called as the shank that surrounds the bearings of the pinion shaft. In non-rotary thrusters the shank is a part of the thruster frame, too. The shank is also a closed cavity, as the pinion shaft bearings require lubrication. The lower gearbox is normally fastened to the bottom of the shank.
The lubrication of the steerable thruster has been arranged this far by either arranging full oil bath in both the stembox, the shank and the lower gearbox or arranging splash lubrication in each one of these lubricating positions.
Thus, it is evident that the cavities within the thrusters used in marine vessels may be either filled with oil or provided with an oil surface, which is, at least in the lower gearbox, normally at about the level of the axis of the propeller. Such oil compartments have, in principle, two problems that have to be taken into consideration when aiming at ensuring a problem-free and safe operation of the thrusters.
Firstly, all compartments having an air space have a tendency of collecting water from the humidity of the ambient air entering the compartment. For instance, when the temperature of the oil in the lubrication system changes, the volume of the air space changes, whereby the air space has to be vented. In case the oil gets cooler outside air flows in the air space, and if the air is humid, the water condenses in the oil compartment. Also, when the oil compartment, like that of a lower gearbox, is below the sea level, the shaft seals try to prevent both the oil from leaking to the sea and the sea water from entering the oil compartment. However, as the hydrostatic pressure acting on the seals from the sea is relatively high (the lower gearbox may be at a depth of several meters) the seals will eventually start to fail, whereby the oil compartment will sooner or later receive small amounts of sea water. The presence of water in the oil weakens the lubrication capability and causes corrosion in the metal parts of the oil compartment as well as those of the entire lubrication system. The current method of removing the water from oil applies additional expensive filters.
Secondly, a natural characteristic of lubrication oils, i.e. the hydrocarbons is to slowly evaporate. In other words, the lightest elements separate in gaseous form in the gas cavity of the oil compartment. If the same oil compartment contains oxygen, there is a risk of explosion if or when the concentration of the gaseous hydrocarbons gradually grows high enough.